High affinity, stereospecific recognition sites (receptors) for putative neurotransmitters, neuromodulators, and many psychotherapeutic agents have been identified in the mammalian central nervous system. It is currently thought that the interaction of a neurotransmitter, neuromodulator, or psychotherapeutic agent with these sites initiates a series of events resulting in either a physiologic/behavioral response (in the case of neurotransmitters and neuromodulators) or a pharmacologic action (in the case of a psychotherapeutic agent). The high affinity, stereoselective binding of psychotherapeutic agents to brain also suggests that previously undescribed endogenous modulators which physiologically mimic (or antagonize) the actions of these agents may also be present. Several receptor systems are currently under study, including: a) "Brain-type" benzodiazepine receptors, believed to play a role in the pathophysiology of anxiety, seizure disorders, musculoskeletal disorders, and sleep disorders. These receptors are part of a "supramolecular complex" which contains recognition sites for benzodiazepines, gamma-aminobutyric acid, and a chloride ionophore. b) The "peripheral-type" recognition sites for benzoidiazepines that are found in both central and peripheral tissues. c) Receptors for central stimulants (e.g., amphetamine). d) Receptors for hallucinogens (e.g. phencyclidine), and e) Recognition sites for drugs which regulate calcium flux.